Acids and Bases (TM)

Many scientists have attempted to define and differentiate the properties of acids and bases throughout the centuries. As far back as the sixteenth century, Robert Boyle noted that acids are corrosive, sour, and change the color of vegetable dyes like litmus from blue to red. On the other hand, bases, or alkaline solutions are slippery, bitter, and change the color of litmus from red to blue. The litmus test is still used today to determine whether a solution is acidic or basic.

Later, Svante Arrhenius gave an even more specific definition of acids and bases. He defined acids as compounds that dissociate when they dissolve in water, releasing H⁺ ions along with a negative ion called a counterion. For example, the acid HCl (hydrochloric acid) dissolves into H⁺ and Cl⁻ ions in water.

Similarly, Arrhenius defined bases as substances which release OH⁻ ions (hydroxide) and a positive ion when dissolved in water. For example, the compound NaOH dissolves into Na⁺ (the counterion) and OH⁻ ions in water. His theory also explains why acids and bases neutralize each other. If acids have an H⁺ ion and bases have an OH⁻ ion, when combined the ions will form water. Along with the water, the counterions usually combine to form a salt. For example, when HCl and NaOH are combined, the result is water and table salt (H₂O and NaCl).

Thomas Lowry and J.N. Bronsted later presented a revised theory of acids and bases. In the Bronsted-Lowry definition of acids and bases, acids are defined as proton donors and bases as proton acceptors. An acid and base are always paired as reactants. The base reactant produces a conjugate acid as a product, paired with a conjugate base produced from the reactant acid. Water, often involved in these reactions, can either accept or donate a proton, meaning that it can act as either an acid or a base, depending on the particular equation.

In the example below, acetic acid (CH₃CO₂H) is dissolved in water, producing a conjugate base (CH₃CO₂⁻). Water acts as the base, and its conjugate acid is the hydronium ion (H₃O⁺).